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a_1)}{2} [/tex]
n= \frac{a_n-a_1}{d} +1

1. There is obviously 50 terms here.
S_n= \frac{n(a_n+[tex]S_{50}= \frac{50(50+1)}{2} =25*51=1275

2. The first term would be 4, and the last would be 80. The common difference is 4.
n= \frac{80-4}{4} +1= 20
S_{20}= \frac{20(80+4)}{2} =840
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